The invention relates to the high temperature transfer of toner between an electrostatic imaging member and a toner receptive surface such as paper. More particularly, the invention relates to the sealing of structures to achieve desired resistivity, dielectric and fluid toner release properties for electrostatic printing and copying devices.
As disclosed in U.S. Pat. No. 4,518,468 an aluminum member may be anodized by an electrolytic process to produce thick oxide coatings with substantial hardness and porosity. The pores of the oxide coating can be filed with a material having suitable dielectric and thermal properties necessary for the transfer of toner.
The anodization of a metal, such as aluminum, to form thick dielectric coatings takes place in an electrolytic bath containing an acid, such as sulfuric or oxalic, in which the metal oxide is slightly soluble. The production techniques, properties, and applications of thick aluminum oxide coatings are described in detail in "The Surface Treatment and Finishing of Aluminum and its Alloys" by S. Wernick and R. Pinner, 4th Edition, 1972, published by Robert Draper Ltd., Peddington, England (chapter IX, page 563). Such coatings are extremely hard and mechanically superior to uncoated aluminum. The coatings contain pores in the form of fine tubes, with a porosity on the order of 10.sup.10 to 10.sup.12 pores per square inch. Typical porosities range from 10 to 30 percent by volume.
For improved mechanical properties as well as to prevent staining, it is customary practice to seal the pores. One standard sealing technique involves partially hydrating the oxide through immersion in boiling water, usually containing certain nickel salts, which forms an expanded beohmite structure in the mouths of the pores. Oxide sealed in this manner will not support an electrostatic charge due to the ionic conductivity of moisture trapped in the pores.
U.S. Pat. No. 3,615,405 discloses a fabrication of an electrophotographic oxide surface by impregnating a porous oxide aluminum surface with an "imaging material." Direct contact is required between the imaging material and the conductive substrate over which the porous oxide layer is formed. This is to provide discharge in radiation struck areas so as to form the electrostatic image to be copied. The present invention transfers the electrostatic image directly without radiation induced discharge. In the '405 patent there is no dehydration of the oxide pores prior to impregnation with an imaging material. This results in a likelihood of trapped moisture, which is deleterious to the dielectric properties of the impregnated anodic layer. The disclosed resistivity of the layer is too low to be employed in accordance with the present invention.
A drum coated with an insulating film capable of supporting an electrostatic charge is disclosed in U.S. Pat. No. 3,907,560. However, at the thicknesses disclosed the oxide layer will support a limited maximum voltage. Also the surface transmits any localized strains through the thin film causing deformation of the aluminum substrate.
In U.S. Pat. No. 4,518,468 carnauba and montan waxes are used as impregnates. They are applied as liquids to an aluminum member at elevated temperatures between 120.degree. C. and 180.degree. C. to assure good penetration and sealing of the pores. While producing excellent results at low temperatures, these waxes cannot be used as impregnates where the dielectric layer must be maintained at temperatures above the melting temperature of the toner because toner flow temperatures typically exceeded the melting temperatures of these waxes. Thus, toner flow temperatures substantially impair the toner release characteristics of the wax impregnates.
In the '468 patent, as the toner particles approach the nip, pressure is applied in order to simultaneously transfer and fuse the toner to paper. Pressures of between 250 and 300 lbs/linear inch between the dielectric member and the paper are typically necessary to achieve efficient fusing. To maintain good transfer efficiency it is necessary to skew the rollers. Skewing of the rollers necessitates skewing of the printhead to maintain proper alignment of the print. This results in a reduction in print quality as the dot size becomes variable depending on how much the cartridge is skewed.
There are other aspects associated with high pressure/low temperature transfer of toner. The papers that are typically toned undergo shrinkage at high pressures. This shrinking of toned papers is highly unsuitable for printing applications where the paper must match predetermined paper size specifications. High pressures also cause "blooming" or expansion of the printed characters. This results from the crushing of the toner particles into the paper. Associated with the reduction in print quality caused by "blooming" is the increase in gloss caused by the high pressure fusion of toner to the paper. It is also desirable to minimize gloss.
It is apparent that a substantial improvement in print quality can be achieved if high transfer efficiencies can be maintained using low pressure transfer of toner. The waxes, resins, stearates or epoxies that might normally be used to impregnate a porous member suffer a substantial reduction in dielectric capacity and/or toner release properties at temperatures where the toner can be melted without pressure.
Accordingly, it is an object of the invention to provide the desired dielectric properties in porous anodized members, while permitting lower pressure transfer of the toner. A related object is to melt the toner without pressure prior to transfer, that is, to permit the dielectric member to operate at temperatures above the melting temperature of the toner without substantial degradation of dielectric strength or loss in release properties.